The present invention relates to the treatment of exhaust gases and in particular, it relates to a process and apparatus for treating an exhaust gas containing volatile organic compounds (VOC""s) or malodorous substances such as ammonia by passing it through a microbial carrier packed layer.
Heretofore, the biological treatment of exhaust gases containing VOC""s generated in chemical plants and the like and those containing ammonia generated in facilities for treating raw sewage, plants for composting sludge and the like is a known method. In such a treatment process, the most serious trouble in the operational control of equipment which is highly possibly caused is clogging of the packed layer. This clogging is easy to occur at the part around the inflow opening of an exhaust gas where the concentration of the VOC""s is highest. Further, when the exhaust gas containing nitrides having nitrogen in the molecule such as acrylonitrile, dimethylformaldehyde, ammonia are treated, ammonia accumulates in the equipment, particularly in the circulating water to be used for spraying in the course of treatment. Ammonia not only raises the pH and lowers the capacity of treatment but also causes a malodor by itself. Further, ammonia is oxidized mainly to nitric acid depending on the condition and also, this nitric acid lowers the pH to reduce the capacity of treatment. Therefore, this nitrogen component has to be removed out of the system. The nitrogen component can be removed by replacing the circulating water, but it will cause the use of a large amount of water and the treatment of waste water.
In view of the above described prior art, the present invention has an object to provide a process and an apparatus for biologically treating an exhaust gas containing nitrides which treat a circulating water containing ammonia within the treating system and do not require a large amount of water.
In order to fulfil the above described object, one embodiment of the present invention relates to a process for treating an exhaust gas containing nitrides by passing it through a microbial carrier packed layer while cyclically spraying water thereon, which comprises nitrating a circulating water containing ammonia, reducing the nitrated water to effect denitrification, and cyclically using the resulting water as a circulating water.